1. Field of the Invention
The present invention relates to a navigation system that displays map information and displays a guidance route to a destination on the map, particularly a navigation system that provides guidance that includes congestion information based on received road traffic information.
2. Description of the Related Art
In recent years, navigation systems have been more frequently installed in vehicles for providing guidance in driving the vehicle, so as to make it possible for the driver to more easily reach a desired destination. A navigation system is a system that is capable of detecting the vehicle's current position using a GPS receiver or the like, then obtaining map data that corresponds to the current position from a storage medium such as a DVD-ROM, hard disk, or the like, or through a network, and displaying the map data on a liquid crystal display. In addition to reading the map data that includes the vehicle's current position, from the storage medium or the like, drawing a map image of the area surrounding the vehicle's current position based on the map data, and displaying the map image on a display system, the navigation system displays a superimposed vehicle position mark on the map image, then scrolls the map image as the vehicle moves or keeps the map image stationary and moves the vehicle position mark, so that the driver can tell at a glance the location of the vehicle.
Various types of navigation systems have been proposed that provide congestion information based on received road traffic information. For example, a navigation system disclosed in Japanese Patent Application Publication No. JP-A-10-283589, (paragraphs (0007) to (0020), FIGS. 1 to 5) draws, on a display screen, a map corresponding to a host vehicle position, receives road traffic volume information from an external source, and displays road traffic volume information on the map. The navigation system also receives, from the external source, road traffic volume information that indicates the lengths of congested sections of monitored roads within specific predetermined areas (“meshes” or “grids”), as well as the congestion levels. The navigation system uses the total length of the monitored roads within each area, the lengths of the congested sections, and the congestion levels to compute the road traffic volume in a given area. The navigation system then controls the coloring or brightness levels of the map image of the area in a manner indicative of the road traffic volume.
However, the navigation system described in Japanese Patent Application Publication No. JP-A-10-283589 displays the average road traffic volume, that is, the average congestion level, for an entire area (grid). Therefore, when the map information display area is a wide area displayed on a reduced scale to a specified level or lower, for example, a wide area display that allows an entire metropolitan area to be viewed, many roads within the area (grid) are omitted from the display. It is therefore difficult to judge accurately which roads and which localities within the area have high traffic volume or are congested. Also, because the road traffic volume is averaged, the displayed road traffic volume is not accurate.